In off-shore operations such as oil and gas explorations and productions, transportation, installation and retrieval of marine structures are very common. Such marine structures include off-shore platforms, jackets supporting such platforms, towers for bridges and windmills and other gravity based structures. It is also known that such marine structures are towed to appropriate locations by vessels, barges, hulls and other bodies having adjustable buoyancy as known to persons skilled in the art. These are effectively used for installation of marine structures. Further, such vessels/barges have been effectively used for retrieval of marine structures, once their life span is over or for shifting those structures to other sites of operation.
U.S. Pat. No. 5,607,260 discloses an apparatus and a method for installation and removal of large multi ton deck packages. It teaches the use of two barges which can support a large multi ton load. A variable dimensional truss assembly is supported by the barge and forms a load transfer interface between the barge and the deck package.
The above prior art document, like most prior art techniques uses two parallel barges, which have to be interconnected with substantially large beams. This makes the arrangement less sturdy and a bit cumbersome, for loading and removing the marine structure, on and from the deck of the vessel. That apart, the top portion of the apparatus has height restriction due to the truss assembly. Hence, height restrictions exist in respect of the structure to be towed or installed or retrieved. That apart, positioning of the barges around the structure to be carried or installed or lifted is also another problem.
International PCT publication number WO 00/78604 discloses a vessel, which is a U-shaped pontoon foundation for removal and installation of marine structures. Columns are attached to the pontoon foundation, extending upwards. A tubular rotational beam is fixed to the top of the transverse pontoon and it is engaged with a bracket attached to a side of the jacket leg to be lifted. Then lifting is done with the help of a lifting frame and de-ballasting.
The above document does teach use of a U-shaped pontoon foundation which enables convenient positioning of the vessel, around a platform being installed or a platform being removed. However, the positioning, lifting and removal is dependent on a lifting frame on the top portion of the pontoon foundation, which naturally imposes a height restriction.
Hence, there has been a need for an apparatus for transportation, installation and removal of marine structures, which has a sturdy structure, requires less length of coupling beams for connecting the units of the apparatus and has free space above of the loading zone. There has been also a need for providing a methodology for transportation, installation and retrieval of marine structures which is simple and quick.
The present invention meets the above mentioned needs and other associated needs by providing an apparatus which has at least two basic units of adjustable buoyancy, which are coupled together in a substantially V-shaped form.